1. Field of the Invention
The present invention generally relates to a control apparatus for an AC generator mounted on a motor vehicle (also referred to as the vehicle-onboard AC generator control apparatus), which apparatus is designed to perform on/off control of a current flowing through a field winding of the AC generator by means of a switching circuit. More particularly, the invention is concerned with a vehicle-onboard AC generator control apparatus which is designed to suppress an erroneous control of the switching circuit due to a leakage current, to thereby prevent the output voltage of the AC generator from rising abnormally.
2. Description of Related Art
For having better understanding of the present invention, the background art will first be reviewed briefly.
FIG. 7 is a circuit diagram showing a circuit configuration of a conventional vehicle-onboard AC generator control apparatus disclosed in Japanese Patent Publication No 63-20098. Referring to, FIG. 7 an AC generator 1 driven by an internal combustion engine (hereinafter referred to simply as the engine) of a motor vehicle (not shown) includes three-phase armature coils 101 and a field winding 102 composed of field coils disposed in opposition to the three-phase armature coils 101, respectively.
A rectifier circuit 2 for converting the output voltage of the three-phase armature coil assembly 101 into a DC voltage is constituted by three pairs of diodes inserted between a main output terminal 201 and a ground terminal 202 of the ground potential, wherein each of the output terminals of the individual three-phase armature coils 101 is connected to a junction between the diodes of each pair.
A switching circuit 3 for turning on/off (opening/closing) a current flow path or loop extending through the field winding 102 includes a pair of power transistors 301A and 301B connected in the form of a Darlington circuit and a fly-wheel diode 302 which is connected with reverse polarity so as to shunt the current flow loop extending through the field winding 102.
The fly-wheel diode 302 has an anode connected to the collectors of the power transistors 301A and 301B, while the cathode of the fly-wheel diode 302 is connected to the main output terminal 201 of the rectifier circuit 2.
On the other hand, the field winding 102 has one end connected to the main output terminal 201 of the rectifier circuit 2 and the output terminal of a battery 5, while the other end of the field coil 102 is grounded by way of a collector-emitter path of the power transistor circuitry 301, whereby a current supply loop through which a field current IF can flow is realized.
A control unit 4 which may be constituted by a microcomputer applies a control signal C to the base of the power transistor 301A constituting a part of the power transistor Darlington circuit 301 incorporated in the switching circuit 3 to thereby control the on/off operation of the power transistor circuitry 301 so that a charging terminal voltage VB appearing at a charging terminal of the battery 5 coincides or matches with a predetermined voltage.
The onboard battery 5 mounted on the motor vehicle (not shown) is charged with a DC voltage output from the rectifier circuit 2 while supplying electric power to the field winding 102 and the control unit 4. A key switch 6 which is turned on (i.e., closed) upon starting of the engine of the motor vehicle is inserted between the battery 5 and the control unit 4 to thereby allow the control unit 4 to be supplied with the electric power from the battery upon starting of the engine operation.
Input to the control unit 4 is not only the charging terminal voltage VB from the charging terminal of the battery 5 but also detection signals output from a variety of sensors (not shown) mounted on the motor vehicle to perform arithmetic operations for the purpose of controlling operations of the engine as well as fittings of the motor vehicle.
Next, description will turn to operation of the conventional vehicle-onboard AC generator control apparatus shown in FIG. 7.
When the key switch 6 is closed or turned on, the control unit 4 is put into operation to output the control signal C in dependence on the charged state of the battery 5.
When the power transistor circuitry 301 incorporated in the switching circuit 3 is turned on in response to the control signal C, the field current IF flows through the field coil 102 along the path or loop formed by the battery 5, the field coil 102, the power transistor circuitry 301 and the ground.
Subsequently, when the AC generator 1 is driven by the engine to start generation of electricity, the three-phase AC voltage is output from the three-phase armature coil assembly 101 and applied to the rectifier circuit 2 to be thereby converted into a DC voltage. As a result of this, the battery 5 is charged with the DC voltage, whereby the charging terminal voltage VB appearing at the charging terminal of the battery 5 increases.
The charging terminal voltage VB of the battery 5 is detected by the control unit 4. When the charging terminal voltage VB increases beyond a predetermined voltage level, the control signal C for the switching circuit 3 is interrupted, as a result of which the power transistor circuitry 301 is turned off.
Consequently, the field current IF flowing through the field coil 102 decreases, involving lowering of the voltage generated by the AC generator 1, which is accompanied with corresponding lowering of the charging terminal voltage VB at the charging terminal of the battery 5.
On the other hand, when the charging terminal voltage VB of the battery 5 lowers below the predetermined voltage level, the control unit 4 outputs the control signal C to turn on the power transistor circuitry 301 to thereby cause the voltage output from the AC generator 1 as well as the charging terminal voltage VB of the battery 5 to increase.
In this manner, the charging terminal voltage VB of the battery 5 is controlled so as to coincide or match with the predetermined voltage level.
In this conjunction, it should be mentioned that the AC generator 1, the rectifier circuit 2 and the switching circuit 3 are usually installed within an engine room to be placed under severe environmental conditions. Consequently, there may undesirably take place a flow of a leakage current IL between an cathode electrode line of the battery 5 from which the charging terminal voltage VB is output and a control input terminal of the power transistor circuitry 301, as indicated by a broken line path in FIG. 7, due to deposition of moisture, dusts and others.
When such leakage current IL occurs, the power transistor circuitry 301 will be turned on (i.e., switched to the conducting state) regardless of whether the control signal C is issued or not, as a result of which the field current IF increases uncontrollably.
When the situation mentioned above arises, the output voltage of the AC generator 1 rises abnormally to bring about overcharging of the battery 5. Thus, not only the battery 5 is subjected to premature deterioration but also electric loads or devices such as head lamps and other electrical devices installed on the motor vehicle may be damaged under application of a high voltage. Besides, the ignition system and other fittings of the engine may be injured to disorder or ultimately stop the operation of the engine.
As is apparent from the above description, in the case of the conventional vehicle-onboard AC generator control apparatus known heretofore, no measures are taken to cope with occurrence of the leakage current IL which flows to the control input terminal of the switching circuit 3. Consequently, the vehicle-onboard AC generator control apparatus suffers serious problems that the output voltage of the AC generator 1 rises abnormally due to erroneous operation of the switching circuit 3 brought about by the leakage current IL to injure not only the battery 5 but also various electric device and fittings of the engine and the motor vehicle.